(a) Field of the Invention
The present invention relates to a composite film which has excellent punching capability and is useful as a bonding element for semiconductor packaging.
(b) Description of Related Art
In semiconductor packaging, a bonding element which is a composite film produced by coating one or both sides of a base film with adhesives is used for bonding chips to lead frames to form LOC (lead on chip) or COL (chip on lead) structures or window-tub structures, or for bonding inner leads to heat spreaders to form composite lead frames with heat sinks attached. The bonding element is generally applied to the lead frames by punching out the bonding element from the composite film over the lead frames and pressing the bonding elements to required parts of the lead frames. However, the yield rate of the resulting lead frames with composite film attached is low since flashes are frequently formed at the punched edges of the composite film.
An object of the present invention is to provide a composite film which is so excellent in punching capability as not to form flashes at the time of punching and are useful as bonding film in semiconductor packaging.
Another object of the present invention is to provide a lead frame with composite film bonded to the required parts of the lead frame by punching the composite film of the present invention and is reliable when used in semiconductor packages.
As a result of studies in the relationship between the properties of composite film and flashes, we have found that flashing can be effectively prevented when there exists specific relation between the thickness T of composite film and the edge tearing strength R and have completed the present invention based on the finding.
That is, the present invention provides a composite film, comprising a base film and an adhesive layer on at least one side of the base film, the composite film having a thickness of T (xcexcm), the base film having an edge tearing strength of R (kg/20 mm), the adhesive layer having a total thickness of A, the base film having a thickness of B, T being related to R by a numerical formula R greater than 0.6T-8 when Txe2x89xa660, or by a numerical formula Rxe2x89xa728 when T greater than 60, and A/B being 0.5 to 1.4.
The present invention further provides a lead frame with a composite film attached, comprising a lead frame body and a composite film applied to the lead frame body, the composite film being punched out from the composite film of the present invention.
The present invention further provides a method of producing a lead frame with a composite film attached, comprising punching out a composite film from the composite film of the present invention, pressing the punched out composite film to a lead frame body, with one adhesive layer contacting a surface of the lead frame body.
Herein, the edge tearing strength of base film is measured according to JIS C 2318.
Flashes, which tend to be formed at the edges of punched composite film comprising a base film and an adhesive layer on at least one side of the base film, can be extremely decreased by using a composite film wherein T and R are related to each other by the above numerical formula and the ratio of the thickness A of the adhesive layer to the thickness B of the base film, A/B, is within the above-described range.
The composite film of the present invention is produced by applying adhesives to one or both sides of a base film, followed by drying. The adhesives applied to both sides of the base film may be identical with or different from each other.
The desirable adhesives are heat resistant adhesives which contain as main components heat resistant thermoplastic resins and have a glass transition temperature (Tg) of 150 to 350xc2x0 C., a water absorption of 3% or less and a broadening length of 2 mm or less, and, therefore, polyimide adhesives and polyamide adhesives are suitable.
Herein, the term xe2x80x9cpolyimidexe2x80x9d means not only polyimide but also any other resin containing imide bonds, such as polyamideimide, polyesterimide or polyetherimide.
If the glass transition temperature is beyond the above-described range, or the water absorption is more than 3% by weight, or the broadening length is longer than 2 mm, the resistance to reflow cracking of packages tends to be decreased.
The water absorption of the heat resistant adhesives is more preferably 2.5% by weight or less, particularly preferably 2.0% by weight or less. The broadening length is more preferably 1 mm or less, particularly preferably 0.5 mm or less.
In addition, fillers, such as ceramic powder, glass powder, silver powder, copper powder, resin powder, gum powder and coupling agents may also be added to the heat resistant adhesives. The heat resistant adhesives may also be used after impregnating with it a base sheet, such as glass fabric, aramid fabric or carbon fiber fabric.
Usable examples of the coupling agents include; vinylsilanes, such as vinyltriethoxysilane, vinyltrimethoxysilane and xcex3-methacryloxy-propyltrimethoxysilane; epoxysilanes, such as xcex3-glycidoxypropyltrimethoxysilane, xcex3-glycidoxypropylmethyldiethoxysilane and xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; aminosilanes, such as xcex3-aminopropyltriethoxysilane, xcex3-aminopropyltrimethoxysilane and N-phenyl-xcex3-aminopropyltrimethoxysilane; mercaptosilanes, such as xcex3-mercaptopropyltrimethoxysilane; and coupling agents such as titanates, aluminum chelates and zircoaluminates.
Of these, silane coupling agents are preferred with epoxysilane coupling agents being particularly preferred.
Herein, xe2x80x9cbroadening length of an adhesivexe2x80x9d defines a width by which a film of the adhesive of 19xc3x9750 mm and 25 xcexcm thick is broadened out perpendicularly from the middle of each side of the film by pressing the film at 350xc2x0 C. at 3 MPa for one minute.
The base film to be used in the present invention, preferably, is a heat resistant film made of an engineering plastic, such as polyimide, polyamide, polysulfone, polyphenylenesulfide, polyetheretherketone or polyarylate. The thickness of the base film is preferably 5 to 150 xcexcm, more preferably 20 to 125 xcexcm.
The glass transition temperature (Tg) of the heat resistant film is preferably higher than the Tg of the adhesives to be used in the present invention, and is preferably 200xc2x0 C. or higher, more preferably 250xc2x0 C. or higher. The heat resistant film preferably has a water absorption of 3% by weight or less, more preferably 2% by weight or less.
Therefore, polyimide film is suitable as the base film to be used in the present invention because of the high Tg, the low water absorption and the low thermal expansion coefficient thereof. The particularly preferred film has a Tg of 250xc2x0 C. or higher, a water absorption of 2% by weight or less and a thermal expansion coefficient of 3xc3x9710xe2x88x925/xc2x0 C. or less.
To increase the adhesion force between the base film and the adhesives, it is preferable to surface treating the base film. The surface treatments applicable in the present invention are not limited and include, for example, chemical treatment, such as alkali treatment and silane coupling agent treatment, physical treatment, such as sand blasting, plasma treatment or corona treatment, and it is possible to select one or more treatments that are the most suitable for the adhesives used. In cases where heat resistant film is used as the base film and adhesive layers of heat resistant adhesive are applied thereto, chemical treatment or plasma treatment is particularly suitable.
The method of forming the adhesive layers on the base film is not particularly limited, and an example of suitable methods comprises applying adhesive varnish to the base film and drying to remove solvents. The method of applying the adhesive varnish to the base film is not particularly limited, and may be any one using a doctor blade, a knife coater or a dye coater. The application may also be performed by dipping the base film in adhesive varnish, but it may be difficult to control the thickness of the adhesive layers.
For example, the adhesive varnish may be a varnish of a heat resistant resin, such as a heat resistant thermoplastic resin, or a heat resistant adhesive comprising mainly of a heat resistant resin, dissolved in solvents. Alternatively the adhesive varnish may be a varnish of a heat resistant resin precursor which can be converted into a heat resistant resin by heat treatment or the like following to coating, or an adhesive composition comprising mainly of the precursor, dissolved in solvents. Examples of the precursors are polyamic acids, which can be converted into polyimides by heat treatment.
When adhesive varnish applied to the base film is subjected to heat treatment for the removal of solvents or for conversion into imide, polyamic acid varnish and polyimide varnish are treated at different temperatures. Polyamic acid varnish is preferably treated at a temperature not lower than the Tg of polyimides to perform the conversion into the polyimides, while polyimide varnish may be treated at any temperature sufficient to remove solvents.
After the adhesive layers are formed on the base film, it is preferable to conduct heat treatment at a temperature of 250xc2x0 C. or higher for 1 to 30 minutes to improve the adhesion force between the adhesive layers and the base film.
The thickness of each adhesive layer formed on the base film is preferably 5 to 50 xcexcm, more preferably 10 to 30 xcexcm.
The lead frame of the present invention comprises a lead frame body and composite film applied to the lead frame body, the composite film being punched out from the composite film of the present invention.
The lead frame bodies may have any structure and, for example, comprise inner lead portions to be connected to a semiconductor chip, outer lead portions to be connected to external circuits, and the composite film of the present invention is applied to predetermined portions of the lead frame bodies.
The lead frames with composite film attached may be produced by bonding the composite film to lead frame bodies by punching the composite film onto the lead frame bodies. Semiconductor packages produced by using the lead frames with composite film attached are excellent in reliability.
For example, the bonding of the composite film to lead frames may be efficiently performed by placing the composite film over a lead frame body, with one adhesive layer facing the lead frame, punching the composite film into strips by using punching metal molds, which continuously press the punched out strips to predetermined portions of the underlying lead frame body at a pressure of 0.1 to 10 MPa for 0.1 to 5 seconds, to bond them by their adhesive layers. The forms of the punched out strips of the composite film vary, for example, depending on the forms of chips, the locations of pads on chips or the designs of lead frames. At the time of punching, the lead frame body is generally heated to a predetermined temperature, for example, 200 to 500xc2x0 C. The composite film to be punched may also be heated. If flashes are formed around the punched strips, the flashes adhere to bonding areas of lead frames, to interfere wire bonding.
Hereinafter, the present invention will be described in detail referring to examples, but the scope of the invention is not limited by the examples.